android9.0-ndk开发（2）-JNI代码

注1：android9.0-ndk开发系列文档提供一个ndk编译JNI库，并访问第三方库，APP 直接通过jar接口调用的方法。
注2：本文为个人学习记录，可能存在个别或多处错误，欢迎指正和讨论。
android9.0-ndk开发（1）-第三方库的生成
android9.0-ndk开发（2）-JNI代码
android9.0-ndk开发（3）-Jar打包
android9.0-ndk开发（4）-APP 调用实例

一、创建java 接口代码

myjni.java

package com.sample.jnitest;

public class myjni {
    public static native int SetString(String msg);
    public static native String GetString();
    //load library
    static
    {
        System.loadLibrary("native_jni_test");
    }
}

点击Make Project
会自动生成.class 文件：
C:\Users\alex\AndroidStudioProjects\jni_test\app\build\intermediates\javac\debug\classes\com\sample\jnitest\myjni.class

二、自动生成JNI头文件

创建jni文件夹
使用Android Stutio的Terminal 切换到jni目录
使用命令自动生成头文件：

javah -d . -classpath C:\Users\alex\AndroidStudioProjects\jnitest\app\build\intermediates\javac\debug\classes com.sample.jnitest.myjni

生成的头文件如下：
com_sample_jnitest_myjni.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include 
/* Header for class com_sample_jnitest_myjni */

#ifndef _Included_com_sample_jnitest_myjni
#define _Included_com_sample_jnitest_myjni
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_sample_jnitest_myjni
 * Method:    SetString
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_com_sample_jnitest_myjni_SetString
  (JNIEnv *, jclass, jstring);

/*
 * Class:     com_sample_jnitest_myjni
 * Method:    GetString
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_com_sample_jnitest_myjni_GetString
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

三、第三方库导入

在jni目录下新建thirdlibs\libs
thirdlibs\include 目录。

将（1）中生成的第三方库so库文件（包含ABI文件夹）复制到libs目录
将（1）中的sample.h文件（包含ABI文件夹）复制到libs目录

四、JNI代码实现

创建com_sample_jnitest_myjni.c
实现头文件定义的函数即可。

JNI代码实现方式二：采用RegisterNatives的方式注册到JVM

4.1 代码实现：

//
// Created by alex on 8/27/2019.
//
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#define JNI_CLASSIC
#ifdef JNI_CLASSIC
#include "com_sample_jnitest_myjni.h"
static jstring g_msg;
/*
 * Class:     com_sample_jnitest_myjni
 * Method:    SetString
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_com_sample_jnitest_myjni_SetString
  (JNIEnv *env, jclass clazz, jstring msg)
{
     __android_log_print(ANDROID_LOG_DEBUG,"myjni","Java_com_sample_jni_1test_myjni_SetString...");
     __android_log_print(ANDROID_LOG_DEBUG,"myjni","%s ",msg);
     g_msg = msg;
    return 0;
}
/*
 * Class:     com_sample_jnitest_myjni
 * Method:    GetString
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_com_sample_jnitest_myjni_GetString
  (JNIEnv *env, jclass clazz, jstring msg)
{
   __android_log_print(ANDROID_LOG_DEBUG,"myjni","Java_com_sample_jni_1test_myjni_GetString...");

   msg = g_msg;
    return 0;
}

#else
static jstring g_msg;
/*
 * Class:     com_sample_jnitest_myjni
 * Method:    SetString
 * Signature: (Ljava/lang/String;)I
 */
jint Java_com_sample_jnitest_myjni_SetString
  (JNIEnv *env, jclass clazz, jstring msg)
{
     __android_log_print(ANDROID_LOG_DEBUG,"myjni","Java_com_sample_jni_1test_myjni_SetString...");
     __android_log_print(ANDROID_LOG_DEBUG,"myjni","%s ",msg);
     g_msg = msg;
    return 0;
}
/*
 * Class:     com_sample_jnitest_myjni
 * Method:    GetString
 * Signature: (Ljava/lang/String;)I
 */
jint Java_com_sample_jnitest_myjni_GetString
  (JNIEnv *env, jclass clazz, jstring msg)
{
   __android_log_print(ANDROID_LOG_DEBUG,"myjni","Java_com_sample_jni_1test_myjni_GetString...");

   msg = g_msg;
    return 0;
}

static const JNINativeMethod methods[]={
        {"Java_com_sample_jnitest_myjni_SetString","(Ljava/lang/String;)I",(void *)Java_com_sample_jnitest_myjni_SetString},
        {"Java_com_sample_jnitest_myjni_GetString","(Ljava/lang/String;)I",(void *)Java_com_sample_jnitest_myjni_GetString},

};

JNIEXPORT jint JNICALL JNIOnLoad
        (JavaVM *jvm, void *reserved)
{
    __android_log_print(ANDROID_LOG_DEBUG,"JNITag","enter jni_onload");
    JNIEnv* env = NULL;
    jclass cls;
    jint result=-1;
    if( (*jvm)->GetEnv(jvm,(void**)&env , JNI_VERSION_1_6) != JNI_OK)
        return JNI_ERR;

    cls=(*env)->FindClass(env,"com/sample/jnitest/myjni");
    if(cls == NULL)
            return JNI_ERR;

    if((*env)->RegisterNatives(env,cls,methods,sizeof(methods)/sizeof(methods[0]))<0){
        return JNI_ERR;
    }

    return JNI_VERSION_1_6;
}
#endif

4.2 创建 Android.mk 和 Application.mk

在jni目录创建 Android.mk 和 Application.mk
创建Android.mk

LOCAL_PATH := $(call my-dir)

local_c_includes := \
    $(LOCAL_PATH)

local_src_files:= \
    com_sample_jnitest_myjni.c

include $(CLEAR_VARS)
LOCAL_LDLIBS +=-llog -lz
LOCAL_MODULE :=native_jni_test
LOCAL_SRC_FILES += $(local_src_files)
LOCAL_C_INCLUDES += $(local_c_includes)
LOCAL_MODULE_TAGS := optional

include $(BUILD_SHARED_LIBRARY)

创建 Application.mk

LOCAL_PATH := $(call my-dir)

APP_STL :=c++_static
APP_CPPFLAGS := -frtti -fexceptions
APP_ABI :=all
APP_PLATFORM :=android-28

APP_BUILD_SCRIPT := $(LOCAL_PATH)/Android.mk

五、编译

在Android Studio使用编译命令：

ndk-build NDK_APPLICATION_MK=./Application.mk  NDK_PROJECT_PATH=./

即可生产各硬件版本的库文件：

[arm64-v8a] Compile        : native_jni_test <= com_sample_jnitest_myjni.c
[arm64-v8a] SharedLibrary  : libnative_jni_test.so
[arm64-v8a] Install        : libnative_jni_test.so => libs/arm64-v8a/libnative_jni_test.so
[x86_64] Compile        : native_jni_test <= com_sample_jnitest_myjni.c
[x86_64] SharedLibrary  : libnative_jni_test.so
[x86_64] Install        : libnative_jni_test.so => libs/x86_64/libnative_jni_test.so
[armeabi-v7a] Compile thumb  : native_jni_test <= com_sample_jnitest_myjni.c
[armeabi-v7a] SharedLibrary  : libnative_jni_test.so
[armeabi-v7a] Install        : libnative_jni_test.so => libs/armeabi-v7a/libnative_jni_test.so
[x86] Compile        : native_jni_test <= com_sample_jnitest_myjni.c
[x86] SharedLibrary  : libnative_jni_test.so
[x86] Install        : libnative_jni_test.so => libs/x86/libnative_jni_test.so